Specific molecular recognition is a fundamental process, being the basis of enzyme-ligand interactions, antibody-antigen interactions and the binding of molecules to receptors. Molecular recognition is achieved through non-covalent interactions such as electrostatic interaction (hydrogen bonds) and hydrophobic interactions. Thermodynamic measurements of binding constants and free energy, enthalpy and entropy changes offer insight into the molecular basis of recognition, particularly when coupled with information from X-ray diffraction and, when possible, site-directed mutagenesis.
Direct measurement of the force of interaction has been made by atomic force microscopy (AFM) as well as surface force apparatus. While AFM is capable of measuring bond rupture forces, the technique has the disadvantage that only one measurement can be made at a time. To date, AFM has been used on avidin-biotin interactions (Florin et al, Science, 1995, 264:415), DNA hybridisation (Boland et al, PNAS, 1995; 92:5291), antibody-antigen interactions (Dammer et al, Biophys. J., 1996; 70:2437) and adhesion glycoproteins (Dammer et al, Science, 1995; 267:1173).
Separating biological molecules on the basis of their relative affinities for ligands is a well recognised technique. For example, in affinity chromatography, the components to be separated are passed down a column that contains a specific ligand. The component of interest binds preferentially and strongly to the column and is retained on the column while the other components are removed. The bound material may be eluted off the column at a later stage.
Separation technologies are an important part of many research experiments. Increasing the sensitivity or selectivity of these techniques is desirable.
Kolomenskii et al, J. Appl. Phys., 1998; 84(4):2404–10, discloses surface cleaning and adhesion studies conducted using laser-generated surface acoustic pulses. The pulses were at a low repetition rate (20 Hz) and constant energy. The procedure was conducted in vacuum, and therefore is not suitable for commercial exploitation. An optical microscope was used to detect the removal of particles and it was not possible to distinguish between particles of different size.
WO-A-98/45692 discloses the use of a piezoelectric crystal sensor for determining the formation/dissociation of clathrate hydrates. Kurosawa et al, Chem. Pharm. Bull, 1990; 38(5):1117–20, reports using such a sensor for the detection of agglutination of antibody-bearing latex. WO-A-98/40739 also discloses such a sensor, including a plate on which specific binding entities are immobilised, for use in indicating the presence of cells in a medium. These sensors are used by measuring a change in resonance frequency at constant voltage.
At present, where possible, most viruses are detected by culture of the specimen in cells, since this method is sensitive although time-consuming. Direct detection of viral DNA or RNA in clinical samples can be achieved using PCR and specific primers tailored for the virus of interest. Since PCR involves an amplification step, cross-contamination is a major problem and it is difficult to establish reliable quantitative methods. Other direct methods include electron microscopy, immune electron microscopy, and methods based on antigen detection with enzyme-linked antibodies. These methods are often relatively insensitive and hence require relatively large quantities of the viral particles.